For welding the studs to an underwater structure or to a land structure in the rain, the portable compact stud welding gun commonly used is enclosed by a water-proof case into which air is blown from the exterior to block water flow into the weld zone for effective stud welding.
However, such method of the prior art has encountered various disadvantages such that the operator makes mistakes in the operation, the welding gun becomes disabled or the operator is exposed to a dangerous electric leak due to flooding occurring when the stud welding gun is directed upwards. To overcome such inconveniences, it has already been proposed that, with the welding gun of such type, an outer cylinder is watertightly mounted on the tip of the stud welding gun so as to enclose the stud, then a ferrule and a ferrule retainer are mounted on the open end of said outer cylinder, and a pressurized gas is supplied from the exterior into said outer cylinder so that the gas blows out through a gap between the stud and the ferrule to expel a quantity of water present immediately below the stud through the open end of the outer cylinder and a sponge mass to the exterior in the form of bubbles. Such an underwater stud welding gun is disclosed by Japanese Patent Publication No. 57-146. The underwater stud welding gun disclosed in this Patent Publication is advantageous so far as the quantity of water around a weld zone is expelled by the pressurized gas supplied from the exterior so as to make the weld zone substantially dry, thereby a possible flood is avoided even when the welding gun is directed upwards and the possibility of a flood due to an incorrect manipulation by the operator is also reduced. However, this welding gun requires, in addition to electric wires for welding, control and grounding, an air hose. Thus the operating freedom of the gun is inconveniently reduced. Additionally, the exterior equipment is necessarily of a large size, since a source of a pressurized gas is required.
Furthermore, fine bubbles discharged through the sponge mass enclosing the open end of the outer cylinder to the exterior during operation makes it difficult to observe the situation at the weld zone and the concentric arrangement of the ferrule, the ferrule retainer, the outer cylinder and the sponge mass, in this order, around the stud necessarily makes the gun tip of a relatively large diameter, so that it is impossible to perform the stud welding at intervals less than the radius of such gun tip.
Moreover, when air is used as the gas, the oxygen partial pressure increases as the depth of water increases so that a suitable inert gas must be used, in the place of the air. Such inert gas must be continuously supplied under pressure to the stud welding gun so long as the latter is present in water and this adds to the cost of the welding operation considering the high consumption of the inert gas.